1. Field of the Invention
The present invention generally relates to adhering solid polymeric materials to substrates. More particularly, the present invention relates to an improvement in the surface treatment of a substrate, e.g., one of metal, plastic or ceramic, whereby solid polymeric materials, e.g., polyethylene, are made to adhere more tenaciously to the surface of the substrate.
2. Description of the Related Art
Methods for the application of solid polymers to the surface(s) of the many kinds of substrates, especially those of metal, are well known. Techniques for adherently applying a solid polymer to a substrate have found use in, for example, coating metal pipes and conduits, conductive wires, cables and other electrical components, to resist or retard corrosion. These coatings may also act as insulation and/or provide other useful properties, e.g., improved mechanical performance, durability, abrasion resistance, color coding, etc.
For example, polyethylene is commonly used for wire insulation including insulation of land, underground, and transoceanic cables, sealing electronics, electronic packages, chemical containers, barrels, tanks, bulk storage containers, intermediate bulk containers, double wall tanks, laboratory beakers, filters, and filtration system components, etc. Polyethylene has exceptional mechanical performance characteristics such as abrasion resistance and toughness and is also known for its high dielectric value and resistance to chemical attack. However, the inert chemical properties of polyethylene which make it so useful in many applications hamper its adherence to many kinds of substrates including a variety of metals and plastics that would benefit from a polyethylene coating.
Various methods for coating substrates are known. For example, U.S. Pat. Nos. 4,985,313 and 5,209,987 disclose electrical wires having both an electrically insulating refractory coating and a layer of solid polymer as insulation.
It is also known that copper oxide possesses the ability to improve the adhesion of coatings to a metal surface. For example, it is known in the semiconductor field that the adherence of copper metal of circuitry innerlayers and cured pre-preg layers or other nonconductive coatings may be improved by subjecting the copper to oxidation thus forming a layer of copper oxide (also known as black oxide). Compounds useful in this oxidization process include etchant compositions based on hydrogen peroxide. Such compositions and processes are disclosed, e.g., in U.S. Pat. No. 4,859,281. In U.S. Pat. No. 6,419,784, a predip solution applied to a copper substrate prior to oxidation of the surface of the copper for improved adhesion of a subsequently applied solid polymer coating thereto is said to improve the uniformity of the coating.
Among the methods for applying coatings to substrates is electro-chemical plating, a well known technique for depositing a metal coating on a surface. However, electroplating may not work in some instances where there is incompatibility between the substrate and the electroplating process which can result in the separation of the plating from the underlying material. In addition, plated surfaces typically possess a fairly smooth outer surface which is not ideal for adding additional layers and techniques for roughening an electroplated surface may remove too much plating leaving exposed substrate.
Other methods for applying coatings to substrates include thermal spraying, which generally involves heating particles to a molten or plastic state and propelling them onto a substrate to form a coating. Thermal spraying processes are used extensively in the semiconductor industry, to recondition worn or used parts and to prepare machine parts for extended wear in high friction and abrasive environments and high temperature applications. Both metal and plastic/polymeric coatings may be applied using these spraying techniques.
Yet another method for applying coatings is kinetic metallization as disclosed, e.g., in U.S. Pat. Nos. 5,795,626 and 6,074,135. This method is primarily used for applying a metal coating to a metal substrate without altering the metallurgical properties of either the coating or the substrate.